On September 11, 2001, four planes crashed as part of coordinated terrorist attacks:

• American Airlines Flight 11 crashed into the North Tower of the World Trade Center.
• United Airlines Flight 175 crashed into the South Tower of the World Trade Center.
• American Airlines Flight 77 crashed into the Pentagon.
• United Airlines Flight 93 crashed in a field near Shanksville, Pennsylvania, after passengers revolted against the hijackers.

So, a total of four planes crashed on 9/11 during these attacks, resulting in massive casualties and destruction.